A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The pattern is transferred onto several successive resist layers on the substrate in order to build up a multi-layer structure with the pattern throughout its thickness. It is therefore important to ensure that the pattern in any given layer is exactly aligned with the pattern in the previous layer. The way that successive patterned layers are aligned is by having alignment marks in the layer, these alignment marks being detectable by an alignment beam that is projected by the projection system before the exposure beam is projected to apply the pattern. In order to leave as much space as possible on the substrate for the exposed pattern, the alignment marks are positioned in scribe lanes, which is the part of the substrate that will be sawn to separate the substrate into individual ICs, for example. Alignment marks have, in the past, taken the form of stacked (in several or all the layers) copper areas alternating with dielectric areas.
As lithographic techniques improve and smaller pattern lines are possible, the number and density of active components (e.g. memory cells in ICs) increases. The use of relatively large copper areas in the alignment marks in the scribe lanes means that the size of the scribe lanes is difficult to decrease and so inefficient use of substrate space is inevitable. The scribe lanes therefore consume space on the substrate that could be more efficiently used for active, useful devices, rather than for alignment marks and/or test structures. Alignment marks that are used in the state of the art contain large structures (i.e. large compared to the typical device dimensions). For processing reasons, the alignment mark should resemble the device/product dimensions to guarantee alignment accuracy. Therefore, a sub-segmentation is added to the large areas (e.g. structures 10 in FIG. 2) inside the mark. The typical dimensions of the sub-segmentation can be around or larger than the wavelength of the alignment beam. This can make the areas 10 transparent for the wavelength of the alignment beam; i.e. so that the alignment beam cannot sense the structures and therefore not be used for alignment.
A known alignment system employs an alignment beam of radiation that is radiated by a separate alignment unit and that is incident on a mark, in the form of a grating, on the substrate. The grating diffracts the alignment beam into a number of sub-beams extending at different angles to the normal of the grating. The distinct sub-beams will be directed with a lens of the alignment unit to different positions in a plane. In this plane, structure may be provided for further separating the different sub-beams. The lens system will also be used to finally image the different sub-beams on a reference plate to create an image of the mark. In this reference plate, a reference mark can be provided and a radiation sensitive detector can be arranged behind the reference mark. The output signal of the detector will be dependent on the extent to which the image of the substrate mark and the reference mark coincide. In this way the extent of alignment of the mark on the substrate with the reference mark in the alignment unit can be measured and optimized. The detector may comprise separate individual detectors for measuring the intensity and the aligned position at different orders. To finish the alignment, the reference in the alignment unit has to be aligned to a second reference mark, for example, one provided to the substrate table with the alignment unit. This second reference mark may then be aligned to a mark in the mask using exposure light.
An alternative alignment system is a direct on-axis alignment system that directs an alignment beam directly upon a mark provided on the substrate via the projection system. This beam will be diffracted by the mark on the substrate into different sub-beams and will be reflected into the projection system. After traversing the projection system the different sub-beams will be focussed on a reference alignment mark provided to the mask. The image of the substrate mark formed by the sub-beams can be imaged upon the reference mark in the mask. In this way the extent of alignment of the mark on the substrate and the reference mark in the mask can be measured and optimized. This can be done by using a radiation sensitive detector constructed and arranged to detect the alignment beam traversing the mark in the mask.
Descriptions of these two types of alignment can be found in EP 1 260 870, which is incorporated herein by reference.
Existing alignment marks are formed on a substantially larger scale than the features imaged on the substrate. For example, a box-in-box type marker may have a size of 10 μm or more whereas the minimum dimension of features imaged on the substrate may be 0.1 μm. Thus, when the alignment marks are projected onto the substrate, the light diffracted by the alignment marks in the mask pattern will travel along different paths through the projection optics than light diffracted by the patterned features. The images of the alignment marks will therefore be subject to different aberrations than the images of the mask features and positional errors in the alignment marks may therefore not be the same as the positional errors in the patterned features. This imposes a limit on the accuracy with which overlay errors can be determined. EP 0 997 782 proposes a solution to this problem whereby the alignment mark is formed as a grating having similar line, width and spacings as those of the circuit pattern. Alignment is performed using a television camera image of the alignment mark on the substrate. However, the alignment mark proposed in EP 0 997 782 only exhibits the same positional errors as patterns of adjacent lines and does not provide improved accuracy in measurement of overlay errors for other types of pattern. Furthermore, the alignment mark requires an additional arrangement for determination of its position and is not compatible with existing alignment systems.
A further solution is proposed in EP 1 260 869, which discloses a substrate provided with an alignment mark in a substantially transmissive process layer overlying the substrate, said mark comprising at least one relatively high reflectance area for reflecting radiation of an alignment beam of radiation, and relatively low reflectance areas for reflecting less radiation of the alignment beam; wherein the high reflectance area is segmented in first and second directions, both directions being substantially perpendicular with respect to each other so that the high reflectance areas comprise predominantly rectangular segments.
By segmenting the relatively high reflectance areas of the alignment mark into rectangular sub-divisions, the mark can be arranged to diffract light into similar positions in the pupil plane of the projection lens as structures in the mask pattern. The image of the alignment mark projected onto the wafer will therefore suffer from the same aberrations as the image of the structures of the mask pattern and thus the position of the mark on the substrate is represented by the position of patterned features. Because each part of the high reflectance area is segmented in two directions into rectangular pieces, the whole mark experiences the same aberrations.
However, generally speaking, the intensity of diffracted radiation is distributed over many orders and not all diffraction orders are used for alignment. Using sub-divisions with a feature size larger than the illumination wavelength means that, because many diffraction orders are created, part of the incident light is diffracted to undesired higher orders. Optimizing the intensity in one of the higher orders may imply a decrease of the intensity in the first order. The first order is used during the coarse wafer alignment. Low signal strength in the first order may result in large aligned position errors.
This results in poor wafer quality for the diffraction orders that are used in the alignment by the alignment wavelength. Furthermore, the alignment marks or gratings are placed in separate positions on the wafers, which results in the diffracted signal being influenced by their different environments and different phase depths. This has been shown to lead to a shift in the signal of up to 8 microns, which is a large error on the sub-divided marks that have a period of the order of a few tens of microns.
A further disadvantage with these types of marks is that they each have to be scanned separately prior to alignment. This gives a low throughput of wafers.